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As recently as the 19th century, Western societies lived by an unspoken pact: what happened in the family stayed in the family. Scandal, and even violence, was nobody else’s business and was best hidden from the public so that the illusion of wholesome, happy and healthy family relationships could be maintained.

Gradually, however, cracks began to appear in society’s veneer, and the image of the modern flourishing family gave way to a more nuanced portrait. First, seemingly isolated incidents of child abuse intruded on public awareness, prompting the establishment of a society for child protection in America by 1875, and a similar society in Britain by 1884. The exposure of marital violence was less forthcoming despite the efforts of early women’s movements. Nearly a century passed before the first shelter for battered women opened in England and its founder, Erin Pizzey, published Scream Quietly or the Neighbours Will Hear (1974), which would spread awareness of domestic violence across Europe. The next year, British geriatric physician G.R. Burston described a phenomenon he called “granny battering,” corresponding to what American practitioners had been calling “battered old person syndrome.”

But there was yet another skeleton to be discovered in the family closet. In 1980 a groundbreaking study, known as the National Family Violence Survey, would be published in the United States under the title Behind Closed Doors: Violence in the American Family; it would reveal that—of all forms of family violence—the most prevalent was that which occurs between siblings.

This discovery remains well worth looking into, because despite reductions in birth rates throughout much of the world, the majority of children have brothers and sisters. Even in China, single-child policies have not been widely implemented outside urban areas. And rather than being immune to sibling violence, single children may suffer indirect but significant effects, since those who are violent toward siblings also tend to be violent toward dating partners and peers.

But what is sibling violence? Don’t all children quarrel and even fight occasionally? Isn’t it an overreaction to classify aggressive behavior between siblings as violence? Where is the line between “normal” sibling conflict and abusive behavior, and how and when should parents intervene?

Oddly enough, despite the surprising finding of the National Family Violence Survey, these questions went largely unconsidered for some time by researchers in their understandable zeal to study forms of domestic violence that had already been accepted as social problems. Sibling violence was easily shrugged off as little more than an exaggerated form of sibling rivalry, a concept that in some circles is considered “Darwinian common sense.” Sibling aggression, bullying and even murder, after all, are certainly not unusual in nature, where in some species the wild young compete to the death for the sole right to the survival resources provided by their parents. Thus the terms “sibling” and “rivalry” were nearly inseparable for decades, particularly in Western cultures, forming the basis of perhaps the most popular view of the sibling relationship.

Interestingly, the first crime described in the Bible is fratricide: a jealous Cain kills his more successful brother, Abel, and when asked his whereabouts, Cain disdainfully responds, “I don’t know. Am I my brother’s keeper?” Clearly he thinks not.

Later in the same book, Genesis, a younger brother (Jacob) cheats his older brother (Esau) out of his inheritance, and a few short chapters farther on, Jacob’s older sons throw their paternally favored brother, Joseph, into a pit and conspire to kill him. In a twist that hints at the complexity possible in sibling relationships, one of the brothers, Reuben, hatches a private plan to return and save Joseph, but he is unable to intervene before the others sell the boy into slavery. Joseph ultimately repays his brothers with kindness, saving their lives in the face of drought and almost certain starvation. But it is the early rivalry between them that tends to be remembered, reinforcing commonly held stereotypes about the nature of sibling relationships.

At the other extreme, the relationship between twins has enjoyed a far more benign reputation. Stories abound of seemingly telepathic twins whose bonds—spiritual and physical—were forged in the womb, who enjoy a harmonious mind meld of the kind that is assumed to be forever beyond the reach of ordinary siblings not fortunate enough to be endowed with a real-life mirror image.

The reality, of course, is that twins can be plagued by competitive and even violent relationships just as singletons can enjoy warm, supportive bonds. But it was perhaps not until Stephen P. Bank and Michael D. Kahn published the 1982 edition of The Sibling Bond, a collection of their research into the complexities of sibling interactions, that these polarized stereotypes were seriously challenged by researchers. Further, Bank and Kahn’s contention that parents are not actually a child’s only significant influence inspired later researchers to explore the rich variety of emotional, developmental and other needs that sibling relationships can fulfill, leading many to regard brothers and sisters as potentially important attachment figures—different from parents, but no less influential. As we will see, the findings of these studies in terms of the influence of sibling relationships on personality development and even resilience raise serious questions about whether and how far parents should tolerate conflict and aggression between their children.

The same body of research also underscores that it isn’t enough for parents to discourage negative interactions between siblings. Just because children don’t lash out at one another overtly doesn’t mean they feel warmly toward one another—and it’s the degree of warmth in a sibling relationship rather than the mere absence of negativity that predicts children’s behavioral, emotional and social adjustment. This isn’t to say that children who feel warmth toward one another will never experience conflict, of course; the goal for parents is to help children increase their ability to resolve conflict reasonably quickly and restore an atmosphere of active support. This may require parents to change their expectations: instead of brushing off hitting, name-calling and shunning as harmless behaviors, parents ideally would make it clear that they expect their children to treat each other with warmth and affection, and would reward such behavior when it occurs spontaneously.

Obviously it is in a parent’s best interest to aim for this: few parents take pleasure in presiding over constant squabbling. More important, it’s in the children’s best interest too. Sibling relationships are likely to be the most enduring they will have in their lifetime. Like our parents, siblings are party to our early experiences, but barring unnatural death, they are likely to remain part of our lives much longer, outliving parents by 20 years or more. In addition, if siblings share both parents with us, we will typically have about 50 percent of our DNA in common. That means they are genetically more like us than anyone else on earth other than our parents. Considering that these relationships can contribute tremendously to the stores of resilience that will help carry us through the adverse events that are an inevitable part of life, it makes sense to ensure that they are as supportive and nurturing as possible. Fortunately, research suggests that even though there will always be some elements outside their control, parents are capable of exerting a powerful influence over whether their children will develop positive or negative relationships, either of which can have a lasting effect on the child’s worldview and corresponding mental health persisting long into adulthood.